Furnace for baking ovens



May 23, 1950 w. L.. MUELLER r-:TAL v2,508,866

FURNACE FOR BAKING OVENS Filed April 10, 1945 'IWShets-Sheet 1 fig/- May 23, 1950 w. L.. MUELLER ETAL 2,508,866

FURNACE FOR BAKING OVENS Filed April '10, 1945 fr sheets-sheet 2 .Mzvzegs May 23, 1950 w. l.. MuELLl-RI ETAL FURNACE FOR BAKING OVENS Filed April 10, 1945 7 Sheets-Sheet 3 I Z W E@ .WFM zzaf May 23, 1950 w. L. MUELLER ETAL FURNACE FoR BAKING ovENs 7 Sheets-Sheet 4 Filed Ap'rl y 10, 1945 .w @NDS May 23, 1950 w. MUELLER ET AL 2,508,866

FURNACE FOR BAKING ovENs Filed'April 10, 1945 7 Sheets-sheet 5- il iff/??? 5f v1 May 23, `1950 w. L. MUELLER ETAL FURNACE FOR BAKING ovENs .'7- Sheets-Sheet 6 Filed April 10, 1945 www @1% mlb) NW1 May 23, 1950 w. 1.. MUELLER ETAL. 2,508,866

' FURNACE FOR BAKING OVENS Filed April 10, 1945 Y Y 7 Sheets-Sheet 7 Patented May 23,l 1950 UNI TED PAT FURNACE FOR BAKINGr` @VENS- William L. Mueller, La Grange, Carl J r Rundquist,

Ghicago, and Carl Richard-i Skarin, WesternV Springs,.ll1., assignors to 'IlhePetersen Oven- Gompany; a. company of Illinois,

Application April, 10, 1945,(Se1ia1No, 587,570

61Claims; li

The present invention relates to furnaces for heating` systems of baking ovens or the like, wherein hot gasesor-combustion areV caused to be circul'atedthrough ducts or-other radiating elements within the; baking-y chamber of the oven. MoreV particularly'thisinvention isV directed to improvements in furnaces for oven heating systems of the-type whereinthemajorportion of the combustiongases are-recirculated fromthe ducts or radiating elements-oithe baking oven to the furnace where they arel reheated and 1 admixed with freshr gasesy 'of combustion and again directed into the ducts orl radiating elements in the baking-V chamber; There i'susually associated with such oven; heating systems, a spill damper located in the return4 duct,v adjacent they furnace, toprovide communicationl with the Ystack so as to permit spi-lling off into. the stack a portion of the combustion gases; The amount of combustiongases permitted to bev passedf through the stack preferably approximateand balance the fresh combustion gases generated in the operation of thefurnace.

'I'he furnace embodyingthe present invention is especially adapted for high-ly: eicient operation with' solid fuel, such as coal; preferably Stoker` fed; and-also lends itself'for highly satis.- factory operation- Wheni using eithergaseous or liquid fuels.

Coall fired furnaces: constructed for use with baking ovens have notheretofore proved" entirely satisfactory for numerous reasons, notwithstanding the possibilities oi" effecting substantial economies in operation. Due. to usualfspace limitations in bakeries; coa-l` fired" furnacesy must e necessarily be. relatively/compact in! construction, and' capable. of high thermaroutputg. necessitate ingV relativelyv high operating` temperatures in order to obtain'proper andi eicientfheat; control in the baking chamber ofthev oven. Ithas been. found in the-main that-coal redl .furnaces Whencperating at relativelyvv high. temperatures re.- sults. inrapi'd deterioration-of or damageztothe refractory lining. ascwefllfas; the: clean out andiinspection doors; and# associated metal parts or theiurnace.

Various constructional:` arrangemenissv have heretofore: been proposed in. cmfljrunctionwith siicli--coalIiflrecl` furnaces for eecting ai cooling of the refractory` lining. when.- the; furnace is= opera ating; by thei introduction of outside This arrangement. results in: materially' offsetting; the thermal;V outputv ofthe furnace forv heath-:ugy the baking chamber, andf also: results materially reducing the efliciency of operation ofthe-'ture nace.

We are aware of acertainfurnace ,construction which is constructed" and, arranged for recirculation ofthe returnedLcombustion gases, over the top andy around a small part ofthe upperportion Q f. the.. side Wall. 0i; the combustion .cl'iambenA andi wherein the return., gases 0i combustion are causcdto. be. directed; into the upnerrortion. of the combustion chamber. This' typehof'furnace is. also. unsatisiactnry. for. numerous reasons. First. the. construction.. is. such as, to necessitate the-.use oi refractory linings 0f; 'great thickness. and.. height, and. Weight. necessitating. theV huilde; ing .oieacniurnace Onthefiob with.the.attend.-Y antrelatively. high, initialnst Qi: onstructinn.. Second, furnaces, so constructed.. cannot' be; prac.- tically moved. as. a. unittoanew.locationsuchas is. frequently necessitated., due. to. changea in arrangement. ofv the operating. facilities. 0f. the bakery., Third. while. a. relatirely great mass. of.. refractory. lining is` employed, and. broushtgun to. a high temperature .in Athe .oneratioaotthe fur.- nace, the retained heat of the refractoryisnot ciiicientlv utilized for. heating. 0i the. baking chamber.V Eurtnermore. duc.. tmthis, relatively great.. amountI of. retained heat. in,A the thick4 re.- fractory lining it is not possib1-to obtain-rela: tivelgrapid-change over 0.1., heating Conditions ofthe baking. chamber.. asisfreqnentlynecessary tof-obtain various.. ranges, oibaking .temperatures in the process of baking a variety of different typesandsizesoi food stuffs..

One-of the. objects, of. this .inventionv is 130.12m-, vide animproved. furnace of. the character.. infv dicated which permits utilizing afgreatlureduced quantity and. thickness of refractory lining;for the combustion,A chamber, resulting in a,g1enm-req duction; in... weightT oi the,4 furnace` and attendant reduction in cost tofmanufactule.. l

Anci-,nerf` obiect is to provide., an improved f-urnace-of the character indicated-.wherein recircu-e latedgases!` oi combustonare directedeinto con.-V tactI with s,ubstantial1v:V the entire. .outer surface of. the. reiractory wall-5 ande-thence. into. the vupper portion of the combustion chamber for. admin. turefwith freshlyl generated. combustion gases; so as to.. Permit obtaining greatly. increased thermal eilig:iene-yI for heatingVv off ther baking: chamber iul;1;ile alsoprovidingy amarrangement fonecffecting continuous cooling of the refraCilOW-liningiso as to.A nrcliona the life thereof, asf-.well -asfrmakinej it possible to effect-a relativelyvrapidfhangeover in thermal output in order toi Br'fimiltly'obtain desireiiy heating: conditions of: tlc-.el bak-ing;V- chamber fcrbakina various-.fined A further object is to provide an improved furnace.- otthe charactereindicated recirculated combustion gases from the baking oven are directed into a gas chamber completely surrounding the combustion chamber so that said gases are caused to absorb heat from the refractory lining while at the same time serving to cool said lining.

A still further object is to provide an improved furnace of the character indicated wherein the combustion chamber is completely surrounded by a gas chamber and wherein the combustion chamber is formed with an upright wall of reduced thickness of refractory and a sheet metal dome supported on said wall, and wherein the gas chamber serves to receive the combustion gases returned from the baking oven and said gases are caused to absorb heat from the refractory lining and said dome and are directed through passages in said dome into the upper portion of the combustion chamber for admixture with freshly generated combustion gases for circulation through the heating system.

A still further object is to provide an improved furnace of the character indicated wherein cooled combustion gases from the baking oven are caused to be directed into the combustion chamber in relatively close proximity to the inspection and clean out door of the furnace for protecting the door against excessive heat.

Still another object is to provide a novel stack damper for furnaces of the character indicated for `controlling the heat conditions in the system and by virtue of which it is possible to obtain a relatively rapid change over in heating conditions such as for cooling or lowering of the temperature of the baking chamber of the oven.

Still another object of this invention is to provide an improved furnace of the character indicated Which is of relatively simple, compact form, inexpensive to manufacture and capable of being prefabricated as a unitary structure for shipment as a unit for installation in association with a baking oven.

Other objects and advantages of this invention will be apparent from the following description taken in connection with the accompanying drawings in which:

Figure 1 is a schematic top view of the furnace embodying the present invention, shown in operative association with a portion of a baking oven.

Figure 2 is a schematic elevational view of a portion of the duct system associated with the furnace, taken substantially as indicated at line 2-2 on Figure 1.

Figure 3 is a vertical schematic view of a portion of the duct system associated with the furnace and the baking oven, taken substantially as indicated at line 3--3 on Figure 1.

Figure 4 is an enlarged vertical sectional View through a portion of the stack and the associated ducts of the heating system, showing the novel stack damper construction embodying the present invention, taken substantially at line 4-4 on Figure 3.

Figure 5 is a diagrammatic top plan View of the furnace embodying the present invention.

Figure 6 is a front elevational view of the furnace, taken substantially as indicated at line Ii-B on Figure 5.

Figure '7 is a vertical sectional view through the furnace, taken substantially as indicated at line 'I'I on Figure 5.

Figure 7a is a fragmentary sectional view taken substantially as indicated at line 'Ia- 1a on Figure 7.

Figure 8 is an enlarged vertical section through 4 the inspection and clean out door of the furnace, taken substantially as indicated at line 8-8 on Figure 5.

Figure 9 is a horizontal sectional view through the furnace, taken immediately above the inspection and clean out door at a plane as indicated at line 9-9 on Figure 8.

Figure 10 is a View, part in elevation and part in section, through the inspection and clean out door, taken substantially as indicated at line Ill-II] on Figure 8.

Figure 1l is a horizontal, fragmentary sectional view through a portion of the furnace wall and the inspection and clean out door, taken r substantially as indicated at line II-II on Figure 8.

Referring to Figures 1 to 3 of the drawings, a portion of a baking oven designated at A, is indicated diagrammatically and which it may be understood has its baking chamber heated by hot combustion gases traveling through suitable heat ducts or radiating elements in a manner well understood in the 4vart. The furnace embodying the present invention, and as indicated generally at B, is shown positioned in close proximity to the oven and a coal fired Stoker is indicated generally at C. The furnace is provided with an outlet duct l0 which receives the hot combustion gases from the upper portion of the furnace and directs them to a blower as indicated diagrammatically at I I, for forcibly supplying the hot gases to the ducts or radiating elements in the baking chamber of the oven. The gases as they travel through the ducts or radiating elements in the baking chamber of the oven give up heat and after complete passage through the baking chamber are caused to be redirected back to the furnace through a return duct I4 which communicates with a spill box I5 from which extends horizontally a duct I6 in communication with the bottom portion of the furnace B as will hereafter be more fully described. EX- tending upwardly from the spill box is a duct I'I which is connected at its upper end in a housing I8 of a stack damper. A by-pass duct I9 also extends upwardly from the upper portion of the supply duct i0 and connects into the housing I8 Iof the stack damper adjacent said duct I'I. Connected to the upper portion of the housing I8 is a stack indicated at 20.

Mounted within the stack damper housing I8 is a stack damper plate indicated at 22, and as shown in the full line position in Figure 4, serves to seal off the by-pass duct I9, with respect to communication with the stack 20. This is the position of adjustment of the damper plate when the furnace is operating. When the furnace is to be shut down the Stoker C is adjusted to hold fire control and the blower I I is permitted to operate until the main duct temperature is reduced to a safe condition, after which the stack damper plate 22 is moved from the full line position to the dotted line position, as seen in Figure 4, at which the by-pass duct I9 is in direct communication with the stack 20, and the spill duct I'I is sealed or closed from communication with said stack 20. When the damper plate is in the dotted position, the hot combustion gases produced in the combustion chamber of the furnace are vented directly to the stack. By virtue of this condition, hot gases are no longer supplied tc the radiating ducts in the baking chamber and thereby a rapid cooling of said chamber is effected.

The upper surface of the damper plate is pro- :zum

vidm wdthaaa et which ispivotaycomected: end of ae operating: ug. projecting: thmghe |851 and*A supported andi; guideri im ar sleeveA 253 assocated` Witliay said housing? i8; By virtue: off the damper: construirtion` repmsentedf in Figurel 4f it is` possiblefby tlie operatioa== oil" a. single* control 244 effect propetf contro the supply-of* hat;gases;- tcttle ductslottheebalmg chamber; asA wellasstl'ieventing of the spill gases to the stack, Ifdesifediit may be understood tlattnene may-be! associated withithe operating7 Z' suitable@ connections toecontifol switches: non' contrcil-ling@^the' operation of the? coalimfedf. stolzer C-andl/c'n: the-blowen'll- Theenovel furnace-construction:embodyinthe present -inventiom isaprovided? witlta: combustion cliambev1 indicated? at` 30. comprising arr-s upright refractory lining 3| of' reduced? tliiokness:` Sura rounding4 the; refractory lining'YV at sheet metal jacket :mi the upper l endf ot which: terminates slightly abovetheeupper margin alf edge of thea-reifraetory: lining 3 lf2. The, upper part-ofthe combustion: chamber: is' the'. forno of "a sheetA metal dome; 35 summarizedinfy spacecrelatiomtoeandimmediatei'yf'above.1 thefreftraictorywa'll. plurality of pairscti circumferentially: spaced aparty clips 35' are attached'to tiret-upper marginalivv portion of jacket' andtlelowerf marginali portiinr` of the:l dome'.l 35i: and saidl c'lpslare interconnected by screws-3.8i witltv spacers BSinterposedfbetween thesclipsf asfclean-Iy "seen: in the drawings; sol-that the. lower marginaledge-l ofdnne-y Eflisfsup'- pottedY in spaceapartfrelationnta-thee-too"of the refractorylwall; formingapassagewaww between: said dome and refractory wall, entirely around the combustion-chamber.: The.s topicff the dome and: the upperfpontlom of? they side- Wall thereof are formedfwitlr. agrp-linalty.7 on openings 14E purpose of"- theM passagem-ayim andi the openings 4&2 is to provideifortlie tree entryof the returned combustion gasesa in-tov the-f upper portionn of 'the combustion: chamber for retreating andiy recit# culationthrough thew syste-mf asriwill bepresently described. 'lionfurtherV insure adequate flow of the; returned combustion: gaseslfery entry into-.the combusti-on.chamber.Y the lowerf mangi-nali'edge'`1 of the; dbme is; provided with. a plixralityfiv oi? circumferentialljw spacedlapart? outwardly bent; tangs: as indicatedf at: Mi; as: maybeA clearl'ri seeing ihr' Figure' 'Tafotfthe-"dfrswings;v sofas` tof-provide additional passageways dlthrouglifwhfch thereto-trie@ comib'ustionfgases: may enter* theconibustiontchambera Sin-round'ing:l thecombustion: chamber: 310; in' spaced apart. relation thereto asfmay-'beclearly seen' i'riL-Illigtnfes:Y anal of'. the dnawmgs; isla sheet:y metall housings. 481, thereby: forming@- aff. gas chamber` 4S? compl-etelygsuxrrounding# the; nefifacitoryvwallffandtliicdomesofithefcomhustton,chamber 'Bhefupneffsnrface onthedbmeofifthefcomfbustiiinr:` chairmen` isrigidly oonneeteci; inl spaced apartllrelation' to the of Webhousing: 48 byffaz-plurality off spaceteb'ars or- `rodstl'lff pretemablyweldedlat opposit--endsvtosthef acentt sur faces; ot'the dome..s1ieeit metalli n. Siirroimdingfthhousing: wisa ja'ckeiifil, oimsulatingmatenialgsuclfiaaroclewooia;

The yr doma 351l ofi: tnef. combrxsmn: chamber! is fotmedtvtithfa laterally@extendingfrdoctiextension adapted i to Iliairttev-if2 intoztheupoer end. of the duct: lilliofithaheatmgssystem Th@ retumrdaxct ISES; connectsiintof. sidewall of.: thezfffurnaceg. operncomnmmgomwth gasav chamber 9, at: titev lower enst. tnencci ciirlfmnm-Mguceeaanowoitnieam im Itzwii-Ifnowfbef-mamifestithatthe.heat@generated witlii'nvtlie ccmbusti'onfchamben 30%, in the' form ofi" Het# combustiim` gases; travels upwardly" into tire dome por-tion; thereof-, andv thence"Y intoYv-tle supply ductn HF and then withthe" assistance voi.' the blower I l, said hot gasesrare forcibly-'fedf-into the lleatducts orrradiatingelements in tliebakingf chamber of theoven; As= the combustion gases g-iveup 4their' heat; While` traveling tl'irongl'i the ducts! on radiating elements ofi thefbaking cham-bier; said gases areredirectedfandreturned tether-umane through thereturnductsl |41 and 'IG and* said gases*1 arel caused to enter the gas chamber" 49; at" the bottomy thereof; and travel around-therefractory'walll and partially around and*I over the dome of the combustion chamber and1enter the upper portion of the` combustion chamber" through tlre-r passagewayy 4'0; the apertures 42 in the top'of 'the" dome and thefvent openingsA 45': Said gases asthey'traVel-througlr the gas*A chamber are caused to absrorbl'a' substantial amount" of" heat' from' the refractory' Wall"1 and d'omefso `that" when said' gases enter the'combuschamberA and become admixed with the freshly-generated"combustion gases; theadmxture will'therr'be circulatedn through theI heating "systenr'ofithe b'akingfoyen'.

Byvirtue' of-the substantial hea-tf absorption by the: recirculated gasesA` as' they pass through" the gaschamber; said gases'as they ententhecom'- 'bustion chamber' willnot in any Waydisturbtlie eflciency of' combustion ofthe furnace because said' gasesar'e sufficiently heated soasnot to: effect cnillingfoi the llame the fire box. Dueto this lConstrnotion substantially'theentire oui'lersurfac'e of 'the'combnstion chamber, including the rre'fractorywall and"dome;. gives up heatto' the-recirculatedgases returning fromthe oven and thussaid refractory' wall isA continuouslycooledX so. that it is now possible for the rst time to' greatly reduce the" thickness' ofthe` refractory lining while-ob'- 'tai'ning' maximum.l thermal eiiiciency ofthe fur'- nace;. and atv theH same timer greatly prolonging the'lif otherrefractory'lining: Bythevexpressi'on fredhced'"thicknessas used herein inA relation to the'refractory'Wall'we mean that on; an average the* thickness.' ofi the refractoryY wall' has been reduced approximately soto 5U percent as' compared; to' refractory Walls heretoforeremployedin furnaces of thefgeneral type embodying the present invention. Itwill be apparent that the recirculatedfgascs are constantly being forced through the heating system of'thetbakihg oven and re,- turneo'to, thefurnace for reheating andcontinued. recirculation inthe system, and. the amount of, tha-returned, orA recirculated, gasesy which are Byppassec throughthe spill'damper for. discharge thnonghtle stack preferably is jist sufficient .to Balance theprimary, combustioninthelfnrnaceefalpinsother. Words, sncient. to. accominodate` the -volumetricincnease OfgasE in thev system.,.nec.essary .tovsupport proper.. combustion.. in the combustion chamber.M

By virtue ofthe seduced-thickness of lretirar-:tory u., Wall and the. continuous andrapid absorption of hea-from therefractory bythe recirculated, gases -itiapossible to rapidly` eiiecty a4r change over` in .heatingconditiifms-fofY the baking-chamber, such as loweringiol thtemperatu-reior. the baking of zo: various types't and-.- sizes oi food; stufscv Such change: over:l may be: rapidly@ effectedy by merely adusting; the temperature indicating; contnolon the: baleingoven.

By@ the constructibnzahove; describen.. 'lin possiblesv produce`V ouz: noyer f fmrnacezcome ramasse struction as a prefabricated, unitary structure for shipment as a complete assembly for installation in a bakery. Thus great economies are effected in manufacture by virtue of prefabrication and by virtue of the greatly reduced amount of weight of the refractory lining.

Furnaces of the general type embodying the present invention are provided with inspection and clean out doors as indicated at 69 in Figure 6 of the drawings. It is well known that due to the excessively high temperature necessary in. operating coal fired furnaces of the type for heating of baking ovens, such doors and other associated metal parts are usually subject to rapid deterioration. Moreover in prior furnaces of this class it has not been found convenient or practical to make an inspection of the interior ofthe furnace during the time of operation thereof due to the excessive heat of the furnace.

In the furnace embodying the present invention, we employ a novel construction by virtue of which the door B is continuously cooled during operation of the furnace. The door opening as indicated at Bl, and as may be seen in Figures 8 and 10, is lined with refractory brick, and is of generally rectangular form. Mounted in surrounding relation to said opening is a conventional metal door frame 63 to which is hingedly connected a door t5, carrying a refractory panel 6B, registering with and of a size corresponding to said opening 6I, as clearly seen in Figure 8. The side walls of the opening have certain of the refractory brick spaced apart to form a plurality of relatively narrow passageways 58, which are in open communication with vertical vducts 69 formed of sheet metal plate 'iii and located at each side of the door opening. Said plates 'iii are each provided with a plurality of apertures 12, in registration with the annular gas chamber 49, so that a portion of the recirculated gases as they are returned to the gas chamber of the furnace are caused to flow through the openings 12 into the ducts 69, and through the passageways 68, into the area of the door opening, immediately adjacent the door 65, so as to eiiect a continuous cooling of the gases adjacent said door. The path of travel of the gases from the gas chamber 49 into the area of the door opening is clearly indicated by the arrows in Figures 8 and l1 of the drawings. By virtue of the continuous flow of comparatively cool returned combustion gases in close proximity to the inspection and clean out door, the temperature at the door is maintained comparatively low, thus greatly increasing the life of the door, and associated metal parts, and making it possible for convenient inspection of the interior of the combustion chamber when the 'furnace is in operation. As may be clearly seen in Figure 10, the refractory lining surrounding the door opening 6I is arranged so as to provide a horizontally extending duct 14, above said opening, communicating with the vertically extending ducts 69 at the sides of the opening, and serving to further assist in cooling the refractory lining immediately adjacent the door opening.

Thus by the furnace structure embodying the present invention it is now possible to efficiently heat the baking chamber of an` oven by the hot gases from a solid fuel fired furnace, and wherein the furnace may be quickly controlled so as to obtain a desired range of thermal output, comparable to that when using gas or fluid fuel. While we have herein shown the furnace in close association to the baking oven, it will be manifest thatit could, if desired, be located remotely therefrom, such as in another room or-on another f floor, while still serving to provide eiiicient heat'- ing of the baking chamber of the oven.

Although we have herein shown and described certain preferred embodiments of our invention, manifestly it is capable of modification and rearrangement of parts without departing from the spirit'and scope thereof. We do not, therefore, wish to be understood as limiting this invention to the precise embodiment therein disclosed, except as we may be so limited by the appended claims.

We claim as our invention:

l. A furnace for heating baking ovens or the like by circulating hot combustion gases, comprising, upright refractory walls bounding a combustion chamber open at its upper end, a sheet metal dome mounted on the upper end of said refractory walls and above the open end of said combustion chamber, said dome having a top wall and a surrounding side wall, said dome substantially bounding a mixing chamber, the lower edge of said dome being spaced from the upper edge of said upright refractory walls, duct means leading from said dome to the oven, a housing' surrounding said upright refractory walls and said dome in spaced relation therewith forming a gas passageway around said upright refractory wall and around and over said dome, return duct means leading from said oven through said housing to communicate with the lower end of said gas passageway, and the lower edge of said dome having portions cut and bent so that a plurality 'of tangs are formed which protrude into the gas passageway so as to direct the ambient gases in the passageway into the chamber bounded by said dome.

2. A furnace for heating baking 'ovens or the like by circulating hot combustion gases, comprising, upright refractory walls bounding a combustion chamber open at its upper end, a sheet metal dome mounted on the upper end of said refractory walls and above the open end of said combustion chamber, said dome having a top wall and surrounding side wall and substantially bounding a mixing chamber, the lower edge of said dome being spaced from the upper edge of said upright refractory walls, duct means leading from said dome to the oven, a housing surrounding said upright refractory walls and said dome in spaced relation therewith forming a gas passageway around said upright refractory wall and around and over said dome, return duct means leading from said oven through said housing to communicate with the lower end of said gas passageway, and the lower'edge of said dome and the upper edge of said refractory walls dening a passageway communicating .between said gas passageway and said mixing chamber.

3. A furnace for heating baking ovens or the like by circulating hot combustion gases, comprising,- upright refractory walls bounding a combustion chamber open at its upper end, a sheet metal dome mounted on the upper end of said refractory walls and above the open end of said combustion chamber, said dome having a top wall and surrounding side wall and substantially bounding a mixing chamber, the lower edge of said dome being spaced-from the upper edge of said upright refractory Walls, duct means leading from said dome to the oven, a housing surrounding said upright refractory walls and said dome in spaced relation therewith forming a gas passageway around said upright refractory wall and around and over said dome, return duct .means-:leading ,from said oven through said housing to communicate with the lower end of said gas passageway, the lower edge of said dome and the upper edge of said refractory walls defining a passageway communicating between said gas passageway and said mixing chamber, and portions of said dome being cut and bent to form a plurality of tangs which protrude into the gas passageway so as to direct the ambient gases in that passageway into the mixing chamber bounded by said dome.

4. A furnace for heating baking ovens or the like by circulating hot combustion gases, comprising, upright refractory walls bounding a combustion chamber open at its upper end, a sheet metal dome mounted on the upper end of said refractory walls and above the open end of said combustion chamber, said dome having a top wall and surrounding side wall and substantially bounding a mixing chamber, the lower edge of said dome ibeing spaced from the upper edge of said upright refractory walls, duct means leading from said dome to the oven, a housing surrounding said upright refractory walls and said dome in spaced relation forming a gas passageway around said upright refractory wall and around and over said dome, return duct means leading from said oven through said housing to communicate with the lower end of said gas passageway, the lower edge of said dome and the upper edge of said refractory walls dening a passageway communicating between said gas passageway and said mixing chamber, and the walls of said dome being perforated to provide a plurality of entrances for the ambient gas in the gas passageway into the mixing chamber bounded by said dome.

5. In a furnace for heating baking ovens or the like having a combustion chamber, a mixing chamber above the combustion chamber, a supply duct leading hot gases from the mixing chamber to the oven and a return duct for directing the cooled gases back to the mixing chamber for reheating and recirculation, a device for controlling the temperature of the mixture of recirculating gases comprising, a stack, a by-pass duct connecting the supply duct and stack, a spill duct connecting the return duct and the stack, said `by-pass duct and spill duct joining the stack at adjacent positions, means comprising a slidable damper plate within the stack, said damper plate registering with the two adjacent ducts at their juncture points to the stack, and means for actuating said damper plate to progressively regulate the two ducts joining the stack by simultaneously closing 0E one duct and opening the other, and vice versa.

6. In a furnace for heating baking ovens or the like having a combustion chamber, a mixing chamber above the combustion chamber, a supply duct leading hot gases from the mixing chamber to the oven and a return duct for directing the cooled gases back to the mixing chamber for reheating and recirculation, a device for controlling the temperature of the mixture of recirculating gases comprising a stack, a by-pass s duct connecting the supply duct and stack, a

spill duct connecting the return duct and the stack, said by-pass duct and spill duct joining the stack at adjacent positions, means comprising a slidable damper plate within the stack, said damper plate registering with the two adjacent ducts at their juncture points to the stack, the two ducts at their juncture points to the stack being of the same cross-sectional size, said damper plate being of the same size as each duct so that it completely seals one duct when the other is wholly open, and means for actuating said damper plate to progressively regulate the two ducts joining the stack by simultaneously closing olf one duct and opening the other, and vice versa.

WILLIAM L. MUELLER.

CARL J. RUNDQUIST.

CARL RICHARD SKARIN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,301,324 Schlatter Apr. 22, 1919 1,431,146 Bolling Oct. 10, 1922 1,629,921 Mansfield May 24, 1927 1,841,533 Hopkins et al Jan. 19, 1932 1,857,447 Engels May V10, 1832 1,967,883 Hofmann July 24, 1934 2,113,426 Engels Apr. 5, 1938 2,137,682 Fisher Nov. 22, 1938 2,160,633 Young May 30, 1939 2,164,954 Stephens July 4, 1939 2,231,445 Grapp Feb. 11, 1941 

